If he's having trouble peeing, that's a really serious issue, and you really need to get him to a vet. With 5 cats, it's probably difficult to tell if he's leaving pee clumps, but have you seen him pee? If so, is it a normal amount of pee for him?
If it's constipation, ideally, he'd be eating only wet food. However, if you can't afford to give canned food more frequently, then try to get him to drink more water. Leave several water bowls in different areas around the house, so that he doesn't have to travel far to get to one. You could get a fountain - some cats who don't drink a lot of water like those and will drink more from them. I think they're fairly expensive, but it would be a longterm investment. Or, if it's feasible, maybe you could put only Minnie on an all-wet food diet?
Is he eating normally? Cats who are constipated will generally not eat very much.